1. Field of the Invention
The present invention relates to a ladder acoustic wave filter device, and a branching filter including the ladder acoustic wave filter device.
2. Description of the Related Art
In related art, as filter devices for use in communication equipment such as a mobile telephone, acoustic wave filter devices using acoustic waves are used. Acoustic wave filter devices include a longitudinally coupled resonator-type acoustic wave filter device, a ladder acoustic filter device, and so on. For example, a longitudinally coupled resonator-type acoustic wave filter device is preferably used as a receiving filter device in communication equipment which is required to be of a balanced type. On the other hand, a ladder acoustic wave filter device is preferably used as a transmitting filter device that is required to have high electric power handling capability.
For example, according to Japanese Unexamined Patent Application Publication No. 9-246911 mentioned below, in such a ladder acoustic wave filter device, apodization weighting is applied to an IDT electrode forming a series arm resonator. An IDT electrode forming a parallel arm resonator is configured as a normal IDT electrode. In Japanese Unexamined Patent Application Publication No. 9-246911, it is described that signal transmission characteristics can be improved by adopting the above-mentioned configuration.
In the ladder acoustic wave filter device described in Japanese Unexamined Patent Application Publication No. 9-246911 mentioned above, ripples occurring in the high frequency side portion of the pass band can be suppressed by applying apodization weighting to the IDT electrode forming the series arm resonator. However, there is a problem in that it is difficult to sufficiently suppress ripples in the middle portion of the pass band. Moreover, there is a problem in that insertion loss in the lower frequency side portion of the pass band deteriorates.